First of all, what exactly IS this reaction you're talking about? Lunging? Growling? Biting? It's a problem that you weren't there for the first encounter. Who was? Did the two dogs meet? Or was his reaction the exact same with the first person that had him, that you're able to wittness?

Do you know much about your dogs history? If he was ever attacked or in a multiple dog household?

Dogs who are more dominant tend to be more reactive when a strange dog is approaching. If this dog is approaching quickly, this could easily cause a reaction out of your male. Not to mention if the dog is coming directly towards you both. (does this reaction occur when your male sees the dalmation across the street as well?)
And that reaction is not neccessarily 'aggression' at all.

Does your dog react to bikes as well?

The reaction is lunging and very aggressive barking (that I've never seen him do before). I don't let him get close enough to bite the other dog. But that is the worry I have, if they were to meet off leash would my dog actually attack the other.

My mother was walking him when he first encountered the dalmatian. I was told the dog and his owner came from behind them on the bike and startled both my mom and Kona. I originally thought it has become a dominance thing, Kona scared by the dog at first encounter then now trys to protect my mother from the other dog. It's only been recently that I've been the one walking Kona when he's met the other dog on the trail. Same reaction as described by my mother.

This man and dog are a little strange, I've never seen them off the bike and they never say anything when they go by. The dalmatian has no reaction. As such, we've never had a chance for Kona to meet the other dog in a normal situation off the bike or at a dog park.

We are always on the pathway behind our house when it happens, we haven't seen this other dog anywhere else.

He has no problem with bikes. I take him on the bike from time to time on leash and off leash when mountain biking (around other bikes too) and we also pass kids and adults on bikes when I walk him, with no reaction.

The shelter had no history on Kona. All I know is he was in a shelter for a while in Montreal, then was moved to one in Aylmer and was there for I think 5 weeks. There was no info as to why he was given up.